1. Field of the Invention
The present invention relates to the field of fiber optic transceivers. More particularly, the present invention relates to the use of MEMS technology including tilting plates and grating light valves to modulate an uplink signal by controlling the position or angle of micro reflectors.
2. Description of the Prior Art
In fiber optical communications, a laser is modulated to carry a signal through a fiber optical channel. In “last mile” network applications, however, such as hybrid fiber-coax networks, fiber-to-the-curb, and passive optical networks, it is desirable to have bi-directional communication capability without requiring the use of a laser internal to the terminal node. FIG. 1 illustrates one way of accomplishing this according to the prior art. A source node 102 such as a central office has a laser 108 for transmitting an optical signal over a fiber channel. The terminal node 104, depicted as a building ancillary to the central office, lacks a laser. The downlink signal λdown may be modulated at a frequency of 1 GHz with a bandwidth from 0.5 GHz to 1.5 GHZ. Information can be transmitted on the downlink signal λdown 112, typically in digital format. Because the terminal node 104 lacks a laser source, to bi-directionally transmit as well as to receive information from the terminal node 104, the incoming signal or downlink λdown 112 is passed through a beam splitter 116. Typically, a beam splitter 116 simply consists of a transparent surface at an angle such that a portion 122 of the downlink signal λdown 112 passes directly through the surface of the beam splitter 116, striking the surface of a detector 122, where it is converted to electrical signals. A second portion 124 of the downlink signal λdown 112 is reflected by the beam splitter 116 into a modulator 120.
The portion 124 of the downlink signal λdown 12 reflected into the modulator 120 is then used as a carrier signal for the modulated uplink signal 128, λup The modulator 120 creates an uplink signal 128 for transmission from the terminal node 104 to the source node 102 by imposing a waveform or envelope, typically digital in nature, upon the incoming carrier signal 124. With a carrier signal 124 of 1 GHZ, the modulator will typically impose a modulated signal 128 on the order of 100 MHz upon the carrier signal. The modulated signal 128 is reflected off the beam splitter 116 resulting in a reflected signal 132 which forms the actual uplink signal λup which is transmitted back to the source node 102. Additionally, a portion 130 of the modulated signal 128 passes through the beam splitter 116 and is typically wasted. To prevent incoming laser light λup of the reflected uplink signal 132 from entering the source laser 108, a circulator 110 channels the uplink signal λup to a source detector 106, where it is typically converted to an electrical signal.
Numerous limitations and liabilities attend the modulation of an uplink signal according to this process. First, packaging cost is increased by use of a beam splitter as depicted in the prior art. Moreover, a beam splitter according to the prior art is a “lossy” transmission and reflection medium since the splitting ratio is applied both to the downlink signals and the uplink signals. The splitting of the downlink signal 112 is essential to transmit a first portion 122 of the downlink signal to the detector while reflecting a second portion 124 of the downlink signal to the modulator 120. However, there is no benefit to splitting the modulated signal 128 through the same beam splitter 116. As noted, a portion 130 of the modulated uplink signal 128 passes through the beam splitter 116 and is effectively wasted. Only a fraction 132 of the modulated uplink signal 128 is reflected back into the uplink transmission stream toward the circulator 110. All transmissions therefore, must anticipate and accommodate the lost energy and lost light intensity represented by this waste. What is needed therefore is a method and apparatus for creating a modulated uplink signal that avoids the packaging costs incurred through beam splitters according to the prior art. There further exists a need for a method and apparatus that avoids the waste incurred through channeling a modulated uplink through a beam splitter according to the prior art.